Overheating Question

[SAGE9]

Seems like I have asked a number of questions as of late and I appreciate all of the feedback from you guys. What can I say - I recently became a 1st time owner of a boat.

So here's the background. 1999 Johnson 150. 2 stroke. Oil Injected.

Was out on Monday and the overheat signal sounded after picking up some seaweed. I immediately slowed down, cut the engine and removed seaweed. I restarted the engine and attempted to power up and the signal went off again so I powered down to ~1,500 rpms until signal and red light went off. When I attempted to power up again, engine was shuddering pretty bad - no signal or red light this time. So, I ran the engine at ~1,500 rpms for next 20 mins thinking about the $1G+ bill that was coming my way :eek: and then dropped off passengers expecting long run back home. After that the engine ran fine with no shuddering. Ran engine the next again with no problems.

Question is whether that shuddering is normal under overheat conditions when under load or is it a sign of damage from overheating? Or did engine finally cool off and permit normal operating conditions??

Many thanks!

[SAGE9]

Found out that the power pack in the Johnson 150 has an rpm restricter once the engine over heats. If you try to power up, the power pack restricts rpms to something like 2K which causes the shuddering. If the engine is shut off and restarted, the powerpack resets. Assuming engine has cooled down, then it will power up again.

[DaleH]

Question is whether that shuddering is normal under overheat conditions when under load or is it a sign of damage from overheating? Or did engine finally cool off and permit normal operating conditions??
Couple things going on here ... first, the OMCs system is designed to do 2 things (1) activate the alarm/light and (2) activate the SLOW (Speed Limiting Overheat Warning) mode.

When you shutoff the motor, you reset both alarms. But after you had restarted the OB, and the alarm/light when off again (powerhead was still too hot), you slowed down until the light/alarm reset, but the SLOW mode was also triggered. Even if/when you slow down to cool the p'head ... if you try to get back up to speed, the SLOW mode will still be triggered, as it can only be reset by shutting off the OB. The SLOW mode intentionally alters the timing of the motor so you would NOT want to :eek: put the OB above a certain number of RPMs, usually limited to 1500 or 2000 (I forget which and do not have my manual in front of me).

SLOW mode is only reset by shutting off the motor. OBs actually run cooler at RPMs high enough to open the thermostats, usually 2000 RPMs, but that's at the speed most boats are starting to go past headway speed and heading towards getting up on plane. If encountered again, I would shutoff to reset alarms, clear obstruction, and restart and run at headway speed for 5 minutes to allow the block to cool some, then try getting onto a slow plane.

Yes, you could add a water pressure gauge ... but if you don't monitor it all the time, you'd still end up where you were. From you have written, I believe you simply triggered the SLOW mode and no permanent damage was done to your OB.

[SAGE9]

That is very helpful. Thanks a lot. I'll stay out of the weed beds :brow

[lemaymiami]

I've put together a few boats over the years and have come to believe that a water pressure guage is one of the few essential items that I want on my rigs.... They're not much money and can absolutely save your engine if used properly. After any encounter with weeds or floating debris a quick glance at the water pressure will tell you if you have a problem BEFORE the engine overheats which is handy. When I'm running at night the water pressure is the one guage I really pay attention to since a floating or slightly submerged plastic bag can literally cook your engine. It's nice to have an overheating alarm and a engine management system that should prevent damage, it's even nicer to notice your water pressure's gone to zero before your engine is in trouble.... A quick stop, then reverse for a moment, before going forward again will usually clear any obstruction and allow your motor to cool properly. If that doesn't do the trick, then shutting down, tilting the motor up so that you can clear it by hand is in order. The pressure will come back to normal and you're on your way. Hope this helps.

[SAGE9]

That is helpful Bob. Thanks!